Hypoxic pulmonary hypertension (PH) contributes to the morbidity and mortality of patients with lung and heart diseases. The pathogenesis of hypoxic PH comprises sustained pulmonary vasoconstriction and structural remodeling of pulmonary arteries. The vasoconstriction involves increased activity of the vasoconstrictors endothelin-1 (ET-1) and serotonin (5-HT), and deficient activity of the vasodilator nitric oxide (NO). This mediator imbalance is also implicated in the arterial wall thickening that includes vascular smooth muscle cell (VSMC) growth. The small GTPase RhoA is activated in VSMC by ET-1 and 5-HT, and inhibited by NO, but the role of RhoA and its downstream effector Rho-kinase in the pathogenesis of hypoxic PH is unknown. However, recent advances in the cell biology and systemic vascular pathophysiology of this signal transduction pathway, and our preliminary results, suggest that Rho/Rho-kinase signaling plays a key role in both the sustained vasoconstriction and arterial remodeling of hypoxic PH. Thus, we hypothesize that chronic hypoxia leads to activation of Rho/Rho-kinase signaling which contributes to PH by: mediating sustained pulmonary vasoconstriction, promoting VSMC growth and vascular remodeling, and regulating the expression of genes related to increased activity of ET-1 and 5-HT, and deficient production of NO. We will investigate in catheterized rats, perfused lungs, isolated pulmonary arteries, and cultured pulmonary artery cells if: 1) chronic hypoxia activates Rho/Rho-kinase signaling in pulmonary arteries, 2) Rho/Rho-kinase-induced Ca2+ sensitization of VSMC contraction mediates sustained hypoxic pulmonary vasoconstriction, and 3) chronic in vivo inhibition of Rho-kinase prevents and reverses development of hypoxic PH by suppressing vasoconstriction, vascular remodeling, and the changes in gene expression that lead to increased activity of ET-1 and 5-HT, and deficient production of NO. Our investigation of the mechanisms by which Rho/Rho-kinase signaling is activated and contributes to hypertensive pulmonary vascular tone and structure will provide new insights into the cellular mechanisms of PH. This information may lead to novel and more effective therapy for PH.